The presence of the valuable chemical raw materials referred to as BTX in petroleum and other naphthas has long been recognized and many techniques have been described for recovery of these single ring aromatics from their admixture with aliphatic hydrocarbons of like boiling point. Early practice favored extraction with selective solvents to separate aromatics from aliphatics. The aromatic concentrate so derived is readily distilled to yield benzene and toluene at desired purity. The four C.sub.8 aromatics ethylbenzene (EB) and the three xylene isomers pose a more difficult problem because their boiling points are within the narrow range of 277.1.degree. F. for EB to 292.degree. F. for o-xylene. The great differences in freezing point provide basis for separating C.sub.8 components by fractional crystallization. Extensive facilities have been installed for recovery of p-xylene in this manner, after which the C.sub.8 aromatics mixture lean in p-xylene can be subjected to catalytic isomerization to generate additional desired p-xylene from o- and m-xylene and the product enriched in p-xylene recycled to the fractional crystallization stage. This leaves EB unchanged to accumulate in the isomerizer/crystallizer "loop" unless steps are taken to remove it.
One approach to that problem of EB build-up was the Octafining process which isomerizes xylenes under hydrogen pressure in the presence of platinum on silica-alumina catalyst. Under Octafining conditions, EB is hydrogenated to ethylcyclohexane, isomerized to dimethylcyclohexane and dehydrogenated to xylene. Concurrent conversion of xylene to undesired by-products is such as to result in net loss of xylene despite the conversion of EB to xylene. That history of Octafining is set out in more detail by U.S. Pat. No. 3,856,872 which describes a major advance by conducting the isomerization in the presence of a catalyst in which the acidic component is a zeolite such as ZSM-5. The entire contents of said patent are incorporated herein by reference.
An improved method for preparing C.sub.8 aromatic fractions from reformate and other sources is described in U.S. Pat. No. 3,945,913 and U.S. Pat. No. 4,078,990, the full contents of each being herein incorporated by this reference. According to that technique, heavy reformate or the like, containing hydrocarbons of at least nine carbon atoms is reacted in the presence of hydrogen under conditions to dealkylate aromatics and concurrently hydrocrack paraffins to lower boiling compounds to be later separated by distillation. This "heavy aromatic processing" yields a distillate C.sub.8 fraction low in EB and free of paraffinic hydrocarbons. An advantageous combination of heavy aromatics processing with the vapor phase isomerization of U.S. Pat. No. 3,856,872 is described in patent application Ser. No. 809,231 filed June 23, 1977 now U.S. Pat. No. 4,101,597, the entire contents of which are incorporated herein by this reference. By the process of that application, heavy (C.sub.9.sup.+) aromatics generated in vapor phase isomerization are converted over a suitable catalyst, preferably in admixture with toluene. Effluents of heavy aromatics processing and of the isomerizer are blended for flash separation of recycle hydrogen and processed together through the product recovery steps.
A still further improvement in xylene manufacture is set out in our prior application Ser. No. 914,645, filed June 12, 1978, the entire contents of which are incorporated herein by this reference. Using as catalyst a combination of metal and a zeolite having very low activity and constrained access to the inner pore area of the zeolite (such as ZSM-5), isomerization of xylenes is achieved at very low xylene loss. During conversion there is a small amount of disproportionation, generally less than 1% of the xylene to toluene and trimethylbenzene. Alkyl chains of two or more carbon atoms are split off, thus eliminating ethylbenzene from the stream in the loop. Because the catalytic isomerizer lacks disproportionation activity, such compounds as trimethylbenzene would build up in the loop unless a C.sub.9 ' fraction were withdrawn. However, because the conversion removes side chains of two or more carbon atoms, it can accept the heavy end of a reformate to make benzene from ethylbenzene, toluene from methylethylbenzene, xylene from dimethylethylbenzene and the like.